Flip the statement:
{if $main eq "catalog" and $current_category.category eq ""}<img src="{$ImagesDir}/custom/logo_splash.gif" alt="Tracy Porter" border="0" width="480" height="49" />{else}
<img src="{$ImagesDir}/custom/logo.gif" alt="Tracy Porter" border="0" width="480" height="49" />{/if}